Internal combustion engine speed control, for example engine idle speed control or cruise control in which engine speed is controlled through engine torque adjustment is generally known to present significant control accuracy and stability challenges. Control designers attempt to provide a system that rapidly rejects the wide variety of disturbances incident on the system that perturb engine load and tend to drive engine speed away from a target or desired engine speed.
To provide such disturbance rejection, situational feedforward control has been proposed in which many of the sources of engine load perturbation are analyzed and interpreted to estimate overall engine load. A significant effort is required to calibrate systems employing such complex feedforward control. Furthermore, the processing throughput required to monitor and analyze the many combinations of the sources of engine load perturbation to arrive at a single load estimate significantly burdens the engine controller.
As an alternative to, or in addition to such feedforward control, feedback control has been proposed in which a control parameter is sensed and fed back to a controller which attempts to compensate for deviations in the parameter value away from a desired value. Current control systems with fuel injected internal combustion engines have significant time delays caused by such processes as engine intake manifold filling, fuel delivery, combustion, and rotational dynamics. The delays caused by such processes decrease torque delivery response and thus introduce significant transient error into the engine speed control. A single feedback loop must typically compromise on the extent it compensates for such delays to remain stable and out of saturation. In other words, the ability of the controller to compensate for the significant time delays is limited by any single feedback loop in its structure, as that single loop must compensate for all of the delay effects. The necessary large gain requirements that are associated with any such single loop are unacceptable as they compromise control stability goals.
Accordingly, it would be desirable to provide for engine speed control while avoiding the rigorous calibration and burdensome throughput required with conventional situational feedforward approaches, and while avoiding the compensation limitations of single feedback loop structures.